This invention relates to hot water distribution systems and more specifically to a recirculation pump used to maintain a flow of hot water throughout the distribution system in order to provide instantaneous hot water at each tap. Since the recirculation system needs only to draw a small amount of hot water from the distribution unit in order to make hot water immediately available to the various taps along the distribution system, the recirculation can be achieved with a smaller gauge of pipe than those used to supply the taps and a low throughput pump. Therefore, a low throughput pump with approximately 10 watts of power could be used in the recirculation circuit. However, when a large amount of hot water is drawn through the taps, the recirculation pump is subject to a substantial drop in input pressure amounting to up to ten times the normal working pressure of the recirculation pump. If the pump is not protected by a check valve the resulting backflow causes the recirculation pump to act as a turbine rotating at a much higher speed than the normal rated speed of the pump. Due to the large calcium buildup which may be expected in hot water circuits, the recirculation pump should not use any shaft and bushing with narrow clearance which can seize under calcium deposits. It is preferable to use a pump with a free-tilting spherical rotor-impeller mounted on a small ball-bearing as the one disclosed in U.S. Pat. No. 3,803,432 Laing. This pump and other types with equivalent performance can be damaged by high speed spinning due to backflow. It thus becomes necessary to install a check valve in series with the recirculation circuit to protect the pump. This in turn requires increasing the power of the recirculation pump to 35 to 50 watts in order to overcome the resistance of the check valve This added power may increase the electric consumption between 200 to 300 kilowatt hours per year.
There is need for an improved recirculation unit which would allow for the elimination of the check valve while limiting the effect of any backflow on the low power recirculation pump.